The use of navigation receivers becomes increasingly pervasive in everyday life. It is all the more common that cars' on-board electronics, smart phones, tablets include navigation receivers, and that applications running thereon capture as input information on position and trajectory of the user of the terminal.
Navigation receivers rely on L-Band RF signals transmitted by Medium Earth Orbiting satellites, which are generally included in constellations comprising tens of them to cover most of the surface of the earth, such as the GPS™ (US), Galileo™ (Europe), Glonass™ (Russia) and Beidou™ (China). These constellations are designated under the generic acronym of GNSS (Global Navigation Satellite System).
GNSS carrier signals are modulated by a pseudo-random code and a navigation message that allow calculation of a pseudo-range between the receiver and a specific satellite. With a minimum of four pseudo-ranges, it is possible to calculate Position, Velocity and Time (PVT) of the receiver.
PVT measurements are affected by a number of errors, some of which are intrinsic to the principle of measurement used (e.g. due to the propagation delay variation of the RF signals through the atmosphere—ionosphere and troposphere, due to variations in the orbits of the satellites), intrinsic to the receiver and satellite imperfections (clock biases for instance), or intrinsic to some configurations of the satellites in view at a moment in time (i.e. elevation of the satellites over the horizon; low dispersion of visible satellites—high Dilution of Precision or DOP). A number of corrections may be used to mitigate these errors, with the use of specific processing techniques which are only available to certain types of receivers. For instance, bi-frequency receivers can mitigate ionospheric errors with a gain of precision from a few tens of meters to a few meters, and even better when combined with precise satellite orbits and clocks which then providing Precise Point Positioning (PPP)—a precision of a few tens of centimetres. Differential GPS and Real Time Kinematics solutions provide similar precision from integration of outside information (relative positioning vis-à-vis a number of fixed reference stations with known positions).
It is more difficult to mitigate in a consistent and efficient manner some errors which depend on the position of the receiver, notably when this position is surrounded by a number of objects which reflect and/or fade the navigation RF signals and/or mask a number of the satellites which should be in line of sight (LOS) at a moment in time. In such conditions, often referred to as GNSS multipath environments, the precision of the calculation of the PVT may be quite poor, all other causes of errors being equal, both at the time of acquiring a GNSS signal and at the time of tracking said signal.
In urban canyons (i.e. streets in between tall buildings), multipath will not only increase the error in the determination of the pseudo-range of a satellite (User Equivalent Range Error or UERE), but also the (Geometric) Dilution Of Precision (GDOP or DOP), because the field of view of the antenna will be narrower thus limiting the increase in precision that may come from the use of additional satellites.
The degradation in UERE is due to the signal impairments of the specific satellite which is acquired or tracked by a tracking loop. Tracking of a satellite relies on a maximization of a correlation function between the acquired code signal and a number of local replicas generated by the receiver of the code signals which are specific to each satellite. The correlation functions will be corrupted by multipath and the satellite may not be correctly acquired or may be lost. Even if the signal tracking is still achievable, signal impairments will affect the correlation function's shape, thus degrading the pseudo-range estimation, and the UERE.
A number of mitigation techniques rely on an increase in the number of correlators to improve the performance of the receiver in a perturbed environment. A number of variants of signal processing techniques may also be added, depending on the waveform of the carrier signal. They may improve the quality of the pseudo-range measurements for the satellites in the field of view (FOV) of the receiver but will not improve the number of these satellites in view or the variance of their elevations. Thus, even with the use of complex and costly receivers, the DOP will be poor in any kind of environment where the FOV of the receivers is reduced.
The present invention discloses a solution to overcome the previously cited drawbacks.